The overall objective of this research is to establish which agents and mechanisms are likely to be useful in stimulating the growth and repair of damaged cardiac and skeletal muscle. After more than two decades of study of hormonal control of muscle growth, we conclude that the somatomedins/insulin-like growth factors (IGF's) are the most important anabolic hormones for control of growth, development, and maintenance of muscle tissue. We now propose to continue our work on the actions of these hormones on cultured muscle cells. Emphasis during the renewal period will be on the relationships among the two IGF's, their receptors, and responses of muscle cells to them. We will attempt to define the ways in which specific proteins act through specific receptors to elicit specific responses in muscle cells. Effort will be concentrated on four responses selected as "key actions" of the somatomedins - inhibition of protein degradation, and stimulation of amino acid uptake, cell proliferation, and differentiation. These were chosen because of their wide range of time courses, sensitivities to somatomedins, and probable involvement of intracellular and membrane components. The central experiments will compare concentration dependences of IGF-I and IGF-II actions with displacement of the 125I-labeled hormones from each of the two known IGF receptors in an attempt to establish which receptor mediates which response. Other experiments will characterize differentiation-related changes in receptors and responses, molecular specificities of mitogens secreted by the BRL cells that are our source of rIGF-II, and exploration of possible mechanisms using a variety of inhibitors. In addition, we will continue our efforts to purify the Differentiation Inhibitor, using our newly acquired HPLC and FPLC equipment.